The present invention relates to thyristor protection in general, and more particularly to a circuit for the detection of individual thyristor failure.
In an array of thyristors it is important to detect without delay if any of the thyristors do not operate properly, in order to be able to shut down the entire system before the failure of one thyristor endangers the operation of the others, or puts into jeopardy the overall system operation.
In this regard, it is desirable to be able to sense thyristor failure individually, although it can be sufficient to know that such failure has occurred without the necessity of identifying immediately which thyristor is defective. Such an approach has the advantage of simplicity, reliability and uniformity.
The present invention is particularly applicable to the detection of a thyristor failure due to a short in the device. A thyristor may fail to turn ON and remain OFF because of misfiring under gating control, or because it inherently is no longer capable of being turned ON. This is called thyristor failure in the open mode. A thyristor may also fail because of overvoltage and the occurrence of an arc over the device. The thyristor remains shorted across its main electrodes, independently of any gating or communication condition. In such case the thyristor is said to have failed in the shorted mode. The present invention is with regard to the detection of thyristor failure in the shorted mode. The invention is particularly applicable to a stack of thyristor sharing in common a relatively high voltage. It is important in such instance that failure of one thyristor be detected and that precautionary measures, such as a shut-down, be immediately taken before the entire voltage exceeds the rating of the other thyristors in the stack, in which situation each thyristor could be damaged one after the other.